Transcribing a song by ear can seem like an overwhelming task at first. Most guitarists do not approach it systematically. Often, guitarists will pick a random riff or lick in a song and just try to learn it.

However, taking a more structured approach will make the whole process far easier...

GETTING STARTED:Find a comfortable, well lit, quiet place to work. Have headphones on to hear the sound without any outside distractions. And, make sure your guitar is 100% in tune.

TOOLS:There really isn’t all that much you need to transcribe a song, apart from your ears.
The song in a HQ format is usually the best choice to transcribe from.You will need a good media player that lets you jump to any part of the song you want. There are also specialist song transcription programs which can be a better choice. Most of the tools are not really necessary unless you are transcribing a song with 10+ simultaneous tracks, but other features (like being able to bookmark parts of the track) can be quite convenient and speed the transcription process along.
Transcribe! is one of the better programs for this. It is relatively inexpensive, has all the basic features you need and a convenient layout for accessing those features.
Audacity is probably the best free choice. It is not specifically a transcription program, but you can bookmark sections of the track and change the tempo without changing pitch. The layout is a bit less convenient, since it is actually designed for recording and editing, rather than transcription.

FIGURE OUT THE KEY:The key helps you determine which notes are most likely to be played. It also helps you determine which chords are in the song’s key. Most musicians neglect to actually get the key of the song before transcribing it. Knowing the key of a song can make all the other parts easier to transcribe.Here’s a simple method for finding the key:- Listen to the bass tones. Follow them and test major and minor chords over them.- When you figure you've discovered the key, play every note of the key's scale over the song until you find the notes that fit the best over the top of the track. This will more than likely be the key of the song.There are alternative methods – for example, to recognize the basis of a particular guitar riff without being able to recognize the specific notes instantly, and use that to figure out the key directly since in most cases a prominent guitar riff will be in the same key as the song.Whichever method you prefer for finding the key of a song, make sure you figure it out. Knowing the key will make everything else that follows much easier.

UNDERSTANDING GUITAR SOLOS:The lead instruments are generally the most time-consuming part of transcribing a song. At this point, you should have the rhythm sections and the knowledge of the key completed, (which will make it much easier to begin putting the lead parts in the correct places).Song melodies are not random! As you develop your ear training, it should become easier to recognize specific musical constructs. Most of the transcribing of these parts involves the recognition of the key of the song and what is musically occurring. This will get easier with practice, as you learn to recognize more and more of the common song constructs.If you have extreme amounts of trouble with a song, there are programs that can slow down the song to make it easier to transcribe each note one at a time. RiffStation is a great option for slowing down songs.

SELECTING A 1st SONG TO TRANSCRIBE:It may seem like trying to pick an easy song and build your way up would be a good way to build your transcription abilities. However this is not a very practical method because it is difficult to distinguish what is actually an easy or a difficult song. Most of the difficulty of transcription is distinguishing the subtle nuances that alter pitch.
For example, being able to differentiate between a straight legato, hammer-on, bend or slide during a lead guitar note transition. All of those techniques can be used to go from one note to the next, but each changes the transition sound very slightly.This aspect of transcription is never going away, (no matter how easy the song sounds).
Occasionally, more virtuosic, (difficult to play parts) can actually be easier to transcribe accurately. For example, there are typically only one or two ways to play a 200 BPM passage of sixteenth notes, but a dozen effective techniques for playing 120 BPM passages of eighth notes.

SIMPLE Vs. COMPLEX:Even taking a seemingly simple song, like Joan Jett and the Blackheart’s version of “I Love Rock ‘n’ Roll,” there are still technical nuances to take into consideration. Joan Jett’s guitar part is fairly simple, and usually the one most people imitate.Ricky Byrd’s guitar part, on the other hand, has a decent amount of technical nuance given how simple the song is. Even on a simple song like that, most people trying to transcribe it are going to miss those nuances in favor of a more simple, less accurate version.
Another good example is the riff from “Smoke on the Water” by Deep Purple. It is considered one of the simplest rock riffs by many guitarists, yet virtually no one, outside Ritchie Blackmore, actually can play the technical nuances correctly. That is because the main riff is very specifically played without the pick, which gives those chords a slightly different tone. This is more difficult to notice because you very specifically need the pick for other parts of the song.
Getting the right notes, chords and rhythm for virtually any song is pretty easy. Getting the technical nuances, no matter how easy the song might seem, is where practice and experience come into play.Try to get the basics of the song when you are starting out. As you get better at transcription, go back to songs you have already transcribed and try to get the technical aspects of the song down perfectly. This is a much better approach than trying to arbitrarily determine if a song sounds easy or not.